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thaumaturge 17th December 2011 01:15 AM

Strangest electronics repairs
 
Over the years, I've encountered some truly bizarre electronics problems. I'm certain that others have their own stories to tell as well. Starting this thread as a place to talk about them.

I'll start it off.

One day at a division of Litton in silicon valley the TFT VLF comparator (that we used to dial in all our frequency time bases at the time) lost lock. An old HP 117A showed good WWVB field strength on a separate antenna. Looking at the TFT a test at the antenna connection showed the proper 15VDC supply output to power the active FET antenna element on the roof. Heading up to the roof I measured mere millivolts at the connection to the antenna. Okay, I'm thinking shorted coax.... Dropping down a floor to a utility closet I found a BNC barrel connection splice in wire laying aside and air conditioning duct. Removing supply side I measure 15VDC. What? Remove output side and get no low ohms readings on antenna side. Looking at barrel it is simply black inside. Measure barrel and I get 10 ohms center to outside... Problem turned out to be a VERY conductive fungus had grown in BNC barrel due to condensation from leaning against cold air duct. Scrubbed inside of male ends with triclorethane and replaced center F-F barrel with a new one and heat shrunk tube around it to keep humidity out. Problem solved. A danged conductive fungus! The ten ohm part really impressed me.

Doc

thaumaturge 18th December 2011 04:31 AM

So... no takers on the conductive fungus. Okay, I've got more.

North Atlantic Phase Angle meter. Exotic box. Basically read the difference in gain and phase between a refference and a test signal anywhere between 10Hz and 100KHz. Only this one is absolutely in spec on everything EXCEPT exactly at 100Hz. There it's just out. What the? Scratched my head a long time on that one. Finally found a phase lock frequency range control IC was skipping over an entire band that should have been there. The phase lock on the range below and above was good enough to ALMOST cover the adjacent range, except at the very end of their lock ranges. 100Hz should have been mid band on a range, but it was alternating between the range below and above. Replaced the IC and all was well.
Doc

stratus46 18th December 2011 04:43 AM

Sony used a glue to hold the fiberglass insulator to the bottom of packaged LCR filters used heavily in the BetaCam SP VTR lines. After 20-30 _years_ it becomes conductive and shorts out parts of the filter. After removing the module from the board it can be cleaned off and re-installed without any glue and once again work correctly. I've 'repaired' dozens of these.

They also use that glue to hold rework wires in place after they mod a board with trace cut(s).

G

thaumaturge 18th December 2011 05:18 AM

That's the stuff I was looking for (referring to your post, not the glue.)

Sony has other little tricks that will trip you up. On virtually all their consumer CRT and older projection bigscreen sets their PWM power supply is about the only one that goes UP in frequency when it reaches regulated voltage. They use a DC swamping coil on their transformers to regulate output. Don't know if their studio monitors did the same... as I never had one with a PS failure.
Doc

Conrad Hoffman 18th December 2011 05:19 AM

Worst thing I ever fixed was a GR impedance comparator. To make a 40 hour troubleshooting story short, there was a cracked solder joint on the grid of a tube. Signals passed the capacitance of the crack with no problem, so the instrument sort of worked, but there was a slight phase shift that prevented calibrating it. One touch of the soldering iron fixed it, but figuring out where to touch was a nightmare.

thaumaturge 18th December 2011 01:15 PM

Ooh I still shiver when I think about working on some of Magnavox's early computer monitors with just out Ecofriendly low lead solder. Bittle as heck, a lot of functions still done with discretes. So oodles of tiny suface mount parts held with brittle solder then they mount the flyback right on the motherboard so the slightest jar breaks... something really hard to find! That's when I concluded -again- that consumer electronics was not for me.
Doc

thaumaturge 19th December 2011 12:57 AM

Okay, keeping it going:

A mitsubitshi (sp?) 13 inch color CRT TV. Works great if you power it up cold, but if you turn it off then right back on it emits a horrible buzzing and doesn't light up CRT.... Okay from receiver module signal gets split off to sound circuit and the detected video goes off to a video Jungle sircuit that splits video into H-drive, V-drive and Red, Green and Blue signals. Using an issolation transformer I start probing the jungle. WHAT? Unless fully powered down (or like on a power bump) the jungle chip gets confused and switches the Vertical drive out the HORIZONTAL drive pin! (Basically sending 30hz to flyback). Wasn't a characteristic of all jungle chips, just the one in that set. A new jungle chip cured it. Still got that jungle chip in my junk box.
Doc

indianajo 19th December 2011 02:35 AM

I bought my first color TV from a friendly master sergeant in the Army, but while watching Star Trek the Movie right after the nuclear explosion, or nova, or whatever it was, all the colors went out but red! Stayed that way, too. I didn't have time or resources to fix it in the middle of Kansas, so I gave up TV until I got out. In Jeffersonville, I buy the Sam's Photofact, pull the case off, and start to debug. Zowww!!! it shocked me silly. Turns out the entire rent house was wired upside down, black and little blade were neutral, white and big blade were hot. Thus the steel TV chassis was hot, too. Wasn't even a steel box outside the house at the meter, just a big ball of electrical tape under the eaves for each of the two connections (no third). The fuse box was the other side of the house from the service entrance. After I found another house to buy, the landlady offered it to me for $11000. Sorry, the asbestos siding was a real barrier besides the stupid wiring. Never did fix that TV, finally gave up on TV's, something about 200 electrolytic capacitors and no working scope.

18Hurts 19th December 2011 03:24 AM

I worked on medical equipment for 25 years and seen some crazy stuff

The #1 weirdest (and most entertaining) was a DVD player in a tent hospital in Iraq. They used the player for movies so patients can take their mind off things as the stretchers moved through the Emergency Room. The Australian medic asked me if I could fix DVD players, sure--I'll give it a shot. Had a CT scanner to work on but gave it a minute, threw in a regular music CD which played so I told him I swing through again at 9 PM.

That night I pulled the cover off the DVD player, loaded in DVD and watched the focus scenario as it attempted to read the disc. The laser moved up, moved down then slightly oscillated as it attempted to focus. Figured there was enough dust on the sensor (Iraq has plenty of dust) but the player kept the same problem with slight oscillation of the laser. Pulled the transport out, looked at it with a magnifying glass and found the problem. There was a hair stuck between the laser and the transport. A very short, very curly hair that acted as a counter spring. Removed the hair and it worked fine, the movies were back on. I attempted to sneak out....

The Australian medic was back and very happy that the DVD player was working and called everyone in. He asked me what the problem was and I told him problems with laser focusing. For some reason, he asked me why so I told him about the very short, very curly hair that acted like a spring. Attempted to sneak out again....

He said "Can't blame any of us, nobody in my section has very short, very curly hair!" Uhhhhh....not exactly true....uhhhhh. Hmmm, so I put it this way "In Texas they call them "short hairs" sooo...." The Americans in the audience smiled but Australians don't know what "short hairs" slang converted to in the Queen's English. I attempted to leave again...

The Australian medic was steadfast in none of his medics had short hair. So I basically told him that I was not implying cranial hair--more along the lines of body hair that is located in other areas of the body. Ya know, it's known to be very curly? His eyes lit up and he yelled "Oh, you talking about pubes, Mate!" The entire section roared with laughter, the patients were laughing and it became quite the scene.

Since the world now knew (tent walls don't stop sound) I told him "I'm not sure what kind of movies you watch, but invite me next time"....then I made my escape. In the end, it was a good thing--those patients needed something to laugh about in such a depressing place.

kevinkr 19th December 2011 03:29 AM

18Hurts, that is one great story, and thanks for sharing it with us.. :D


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